The field of wireless data communications represents one of the most rapidly evolving technologies in wide spread use today. Data communications and data processing has become important to virtually every segment of the nation's economy. The demand for efficient and reliable data transmission systems has increased the requirement for the control of errors to enable the reliable reproduction of data.
Information needs to be reliably transmitted and received. This holds true for data communications as well as voice communications. Wireless transmission systems transmit data from a transmitter to a receiver through a communications channel. The communications channel is typically an over the air, RF transmission. Examples include cellular telephony applications, two way radio communications, wireless Ethernet, and the like. Transmission conditions, that is, the degree to which RF signals are distorted by various conditions of the communications channel (e.g., weather, multi-path interference, multiple transmitter interference, etc.) are often problematic. A primary measure of the effectiveness of a wireless communications system is its reliability and performance irrespective of transmission conditions. Reliable transmission should be ensured even in the presence of significant interference, noise, or other problems with the communications channel.
One method for ensuring reliable transmission is to utilize efficient error control and correction techniques (ECC). Modern ECC processes are primarily implemented through error correction code schemes. Error control coding as implemented between a transmitting device and a receiving device incorporates information into a transmitted digital data stream (e.g., a signal) that allows the receiver to find and correct bit errors occurring in transmission and/or storage. Since such coding detects or corrects errors incurred in the communication, it is often referred to as channel coding. The transmitting side of the error-control coding adds redundant bits or symbols to the original signal sequence and the receiving side uses these bits or symbols to detect and/or correct any errors that occurred during transmission. In general the more redundant bits added to the transmitted sequence the more errors that can be detected and more redundancy is required for detection and correction than for detection alone. However, there exists several practical limits to the degree to which an efficient ECC scheme can compensate for problems with the communications channel.
Another method for ensuring reliable transmission is the use of sophisticated noise cancellation and/or filtering processes. Many of these processes utilize sophisticated signal processing schemes to enhance the receiver's ability to filter out the effects of noise within the communications channel.
One such scheme involves the use of multiple transmitter antennas to transmit RF signals to receivers through a given communications channel. Each antenna transmits a version of the outgoing signal in order to cancel out the effects of fading, wherein one version may experience more fading than another version (e.g., due to multipath effects, etc.). This method is generally referred to as transmit diversity.
Transmit diversity can operate using closed loop or open loop methods. Open loop transmit diversity does not use any feedback from receivers to alter signals transmitted from the station. Closed loop transmit diversity utilizes feedback signals from the receivers to alter the versions of the outgoing signal (e.g., from the multiple antennas).
In general, closed loop mode transmit diversity is a well known technique for countering the effects of signal fading. In closed loop mode transmit diversity, to maximize the power of the transmitted signal (e.g., a DPCH signal) received at antenna of mobile station, the antenna weights at base station are generated based on FSM (Feedback Signaling Message) transmitted from mobile station. Since FSM is not protected by any ECC scheme, the bit error rate in the uplink of the FSM bits could be very high.
FSM error causes downlink performance degradation in at least two ways. Due to the FSM error, generated antenna weights at the base station will no longer be at the optimal setting. Received DPCH signals at the mobile station consequently suffer from power loss. And in the mobile station receiver, if the restored antenna weights are different from the real settings in the base station, additional performance degradation will be introduced.
In the mobile station, to demodulate the received DPCH signal correctly, channel characteristics of the DPCH signals (including antenna weights in base station) are required. There are two approaches to get the channel characteristics, one is to estimate it directly by the dedicated pilot symbols in DPCH, and another is combining the channel estimation of CPICH and antenna weights restored at mobile station. Usually the later approach with antenna verification is adopted. Furthermore, antenna verification mitigates the additional performance degradation caused by FSM error. With the former approach, the additional performance degradation can be avoided. However, the total performance is worse than the later one, since in low downlink data rate, the power of CPICH is much high than that of DPCH; in high data rate, the ratio of pilot symbols to total symbols transmitted in one slot is much smaller.
This has the effect of significantly reducing the overall system's ability to operate in the presence of noise and interference. Because feedback information regarding the transmission characteristics of the communications channel between the transmitter and the mobile station is corrupted (e.g., excessive bit error rate), the closed loop correcting process of the antenna weights cannot properly compensate for interference and/or noise.
Thus, what is required is an antenna verification method that can reduce the effects of downlink performance degradation in a closed loop transmit diversity communications system. The required solution should efficiently reduce the effects of FSM error between a transmitter and a mobile station. The present invention provides a novel solution to the above requirements.